Mechanism of inhibition of iodothyronine-5'-deiodinase by thioureylenes and sulfite

ANNIVERSARY REVIEW: Antithyroid drug therapy: 70 years later

The thionamide antithyroid drugs were discovered in large part following serendipitous observations by a number of investigators in the 1940s who found that sulfhydryl-containing compounds were goitrogenic in animals. This prompted Prof. Edwin B Astwood to pioneer the use of these compounds to treat hyperthyroidism in the early 1940s and to develop the more potent and less toxic drugs that are used today. Despite their simple molecular structure and ease of use, many uncertainties remain, including their mechanism(s) of action, clinical role, optimal use in pregnancy and the prediction and prevention of rare but potentially life-threatening adverse reactions. In this review, we summarize the history of the development of these drugs and outline their current role in the clinical management of patients with hyperthyroidism.

Selenium nanoparticles prevents lead acetate-induced hypothyroidism and oxidative damage of thyroid tissues in male rats through modulation of selenoenzymes and suppression of miR-224

Selenium nanoparticles (Se-NPs) are customizable drug delivery vehicles that show good bioavailability, higher efficacy and lower toxicity than ordinary Se. Pre-treatment of male rats with these NPs has been recently shown to exert a protective effect against chromium-induced thyroid dysfunction. This study, therefore, aimed to investigate and characterize the potential protective mechanism of Se-NPs against lead (Pb) acetate-induced thyrotoxicity. We found that prophylactic and concurrent treatment of Pb acetate-exposed rats with Nano-Se (0.5 mg/kg, i.p) for 15 wk significantly alleviated the decrease in free triiodothyronine (fT3) and free thyroxine (fT4) levels as well as fT3/fT4 ratio% and the increase in thyroid stimulating hormone (TSH) levels to approach control values. This was accompanied by a reduction in the accumulation of Pb in serum and thyroid tissues as well as maintenance of thyroidal pro-oxidant/antioxidant balance and iodothyronine deiodinase type 1 (ID1), an essential enzyme for metabolizing of T4 into active T3, gene expression. Surprisingly, miR-224, a direct complementary target of ID1 mRNA, expression in the thyroid tissues was significantly down-regulated in Nano-Se-pre- and co-treated Pb acetate intoxicated animals. Such changes in miR-224 expression were negatively correlated with the changes in ID1 gene expression and serum fT3 level. These results suggest that Se-NPs can rescue from Pb-induced impairment of thyroid function through the maintenance of selenoproteins and down-regulation of miR-224.

Association of Anion Gap with Thyroid Dysfunction and Nodular Goiter in Capd Patients

Objective

To investigate the association of clinical parameters and serum anion gap with thyroid dysfunction and nodular goiter in continuous ambulatory peritoneal dialysis (CAPD) patients.

Design

Cross-sectional study.

Setting

Single dialysis unit and outpatient clinic.

Patients

This study included 89 uremic patients on CAPD. Gender ratio was 50 males to 39 females (M/F = 1.28); mean age was 54.8 years.

Main Outcome Measures

We investigated the prevalence of nodular goiter and thyroid dysfunction with a 10-MHz high-frequency ultrasound scanner and immunoassay kits.

Results

Nodular goiter was detected in 52.8% (47/89) of the CAPD patients. Patients with nodular goiter were older than those without goiter (57.7 vs 51.5 years, p < 0.05). Nodular goiter was found more frequently in females than in males (66.7% vs 44.0%, p < 0.05). Patients with nodular goiter had longer duration of CAPD than patients without goiter (51.6 ± 42.9 vs 31.0 ± 28.1 months, p < 0.02). In addition, CAPD patients with goiter had a higher serum anion gap (AG) (16.8 ± 3.3 vs 14.0 ± 4.5 mEq/L, p < 0.02) and a lower weekly creatinine clearance (55.9 ± 12.6 vs 64.6 ± 21.1 L/week/1.73 m2, p < 0.05) than patients without goiter. As serum AG gradually increased, significant alteration of thyroid parameters developed in the following sequence: ( 1 ) reduction of total T3 level at an AG level of 15, ( 2 ) elevation of thyrotropin (TSH) and increased prevalence of goiter at an AG of 18, and ( 3 ) reduction of free T4 and total T4 levels and elevation of TSH, with further increased frequency of goiter at an AG of 20 mEq/L.

Conclusion

According to this study, age, gender, dialysis duration, serum AG, and weekly creatinine clearance are correlated with prevalence of goiter in CAPD patients. Sequential alteration of thyroid function and increasing frequency of nodular goiter correlated with higher serum AG. There are two explanations for this correlation: the level of serum AG may be an indirect index of the level of serum goitrogens, and higher serum AG and increased frequency of nodular goiters might be a reflection of loss of residual renal function. Therefore, thyroid function screening and goiter detection using ultrasound should be considered when examining CAPD patients with progressively elevating serum anion gap.

Characterization of the subunit structure of the catalytically active type I iodothyronine deiodinase

Type I iodothyronine deiodinase is a approximately 50-kDa, integral membrane protein that catalyzes the outer ring deiodination of thyroxine. Despite the identification and cloning of a 27-kDa selenoprotein with the catalytic properties of the type I enzyme, the composition and the physical nature of the active deiodinase are unknown. In this report, we use a molecular approach to determine holoenzyme composition, the role of the membrane anchor on enzyme assembly, and the contribution of individual 27-kDa subunits to catalysis. Overexpression of an immunologically unique rat 27-kDa protein in LLC-PK1 cells that contain abundant catalytically active 27-kDa selenoprotein decreased deiodination by approximately 50%, and > 95% of the LLC-PK1 derived 27-kDa selenoprotein was specifically immune precipitated by the anti-rat enzyme antibody. The hybrid enzyme had a molecular mass of 54 kDa and an s(20,w) of approximately 3.5 S indicating that every native 27-kDa selenoprotein partnered with an inert rat 27-kDa subunit in a homodimer. Enzyme assembly did not depend on the presence of the N-terminal membrane anchor of the 27-kDa subunit. Direct visualization of the deiodinase dimer showed that the holoenzyme was sorted to the basolateral plasma membrane of the renal epithelial cell.

Thyroid hormone metabolism and nuclear binding in Gunn rats

We examined the serum concentrations of total, free thyroid hormones and TSH, activity of hepatic T4 5'-deiodinase, and T3 binding to hepatic nuclei in homozygous (j/j) and heterozygous (j/+) Gunn rats. Both total T3 and free T3 (FT3) concentrations in sera from j/j rats were significantly lower than those of j/+ rats on 5-10, 15-20, and 25-30 days after birth. Both total T4 and free T4 (FT4) concentrations in j/j and j/+ rat sera were not significantly different on 5-10 days. However, in j/j rats they were significantly higher than those of j/+ rats on days 15-20 and 25-30. Serum reverse T3 (rT3) concentrations were higher in j/j than in j/+ rats on days 5-10, 15-20, and 25-30. Serum TSH concentration in j/j and j/+ rats on 15 days post-natal were 1.42 +/- 1.28 and 1.65 +/- 1.24 micrograms/l (mean +/- SD), respectively, which were not significantly different from each other. T3 formation from T4 in hepatic microsomal fractions obtained 15 days after birth was significantly lower in homozygotes than in heterozygotes (4.89 +/- 1.18 vs 11.15 +/- 2.38 pmol/mg protein/min, p less than 0.005). Binding constants (Ka) as well as maximal binding capacities (MBC) for T3 of hepatic nuclei from 15 day-old j/j and j/+ rats were similar (ka; 3.58 x 10(9) vs 3.15 x 10(9) M-1, MBC; 0.316 vs 0.380 pmol/mg DNA). From these results we suggest that decreased conversion from T4 to T3 is one of the major reasons for high serum levels of T4 and rT3, and low levels of T3 in j/j rats, and that nuclear T3 binding and pituitary TSH secretion are unaltered in j/j rats.

[Effect of iodine, copper and zinc supplements to rations with a high quota of rapeseed extract meal on the growth and thyroid function of fattening swine. 3. The effect on weight and the histomorphometric findings in the thyroid and on the serum T3 and T4 concentrations]

Studies were made with a total of 272 fattening pigs. Iodine deficiency both in rations of soya bean meal and rape seed meal increased the weight of the thyroid gland and the height of the epithelium cells and significantly reduced the T4 and T3 level in the serum. Iodine supplements to the rape seed meal rations distinctly diminished the goitrogenic effect, did not, however, cancel it out. Stage of goiter and depression of consumption were--different shown by the T3 and T4 level in the serum. Highly significant relations were calculated between live weight gain, weight and the height of the epithelium cells on the one hand and the T4 level in the serum on the other. Iodine supplementations of rations without thyrostatic components should amount to greater than or equal to 0.1 mg/kg. Iodine supplementations of 0.5 mg/kg feed are at present recommended to rations with rape seed meal quotas of less than 8%.

Selective modification of the active center of renal iodothyronine 5'-deiodinase by iodoacetate

Pretreatment of renal iodothyronine 5'-deiodinase with sulfhydryl reagents, iodoacetate, iodoacetamide and N-alkylmaleimides, results in irreversible loss of catalytic activity. Iodoacetate and iodoacetamide were the most potent inhibitors, being 100- to 1000-times more potent than N-alkylmaleimides. Iodoacetate and iodoacetamide inactivation followed pseudo-first-order kinetics with maximum inactivation rate constants of 1.56 min-1 and 0.87 min-1, respectively. Thyroxine and 3,3',5'-triiodothyronine and the competitive inhibitor iopanoate, protected the enzyme against iodoacetate inhibition. Protection by 3,3',5'-triiodothyronine was competitive with iodoacetate with a dissociation constant (Kd) of 113 nM; in close agreement with the Km for rT3 of 190 nM determined under similar reaction conditions. [3H]Carboxymethylation of renal membranes in the absence and presence of 3,3',5'-triiodothyronine showed specific incorporation of iodo[3H]acetate into substrate-protected sites of 35-40% of total when non-essential residues were first blocked with excess unlabeled iodoacetate. ' Protectable ' [3H]acetate incorporation followed pseudo-first-order kinetics and the rate constant for incorporation was identical to the rate constant for inactivation. These results indicate that iodoacetate fulfills the minimum criteria for an active-site-directed reagent for renal 5'-deiodinase and that a sulfhydryl group is in close proximity to the iodothyronine-binding site.

Structural requirements of thiol compounds in the inhibition of human liver iodothyronine 5'-deiodinase

2-Thiouracil and a number of its alkyl derivatives are known to inhibit the enzymic 5'-deodination of thyroxine to 3,5,3'-tri-iodothyronine. The structural requirements for inhibition of iodothyronine 5'-deiodinase were investigated by using a washed postmitochondrial particulate fraction of human liver. A series of sulphur-containing derivatives of pyrimidine, pyridine, imidazole, benzene and urea, capable of existing in a thiol form, were incubated at several concentrations with the enzyme preparation in the presence of thyroxine and dithioerythritol (cofactor). The degree of inhibition by the respective compounds of the production of 3,5,3'-tri-iodothyronine was studied in relation to their structural features. The major observations were: (i) a free thiol group is essential; (ii) compounds that do not possess a polar hydrogen atom spatially configured so that it is proximal to the thiol group are poor inhibitors; (iii) aromatic characteristics in the presence of requirements (i) and (ii) lead to the expression of potent inhibitory properties; (iv) modification of potent inhibitors by the introduction of hydrophilic substituents reduces the inhibitory potency.

Inner ring deiodination of thyroxine and 3,5,3'-triiodothyronine by human fetal membranes

Indirect evidence, based on injection of thyroxine (T4) into the amniotic cavity of humans, and maternal thyroidectomy in the rat, suggests that fetal membranes might be capable of converting T4 to 3,3',5'-triiodothyronine (rT3) by virtue of inner ring iodothyronine deiodinase activity. The present study was undertaken to provide direct evidence that human fetal membranes contain inner ring iodothyronine deiodinase activity directed toward T4 and 3,5,3'-triiodothyronine (T3). Homogenates of human fetal membranes were incubated with 125I-labeled T4, rT3, and T3, and with stable T4. Conversion of 125[I]-T4 to 125[I]-rT3 was noted in chorion and amnion. 125I-T3 was converted to 125[I]-3,3'-diiodothyronine (T2) in chorion and amnion. 125[I]-rT3 was stable in fetal membranes under the incubation conditions employed. Time-, temperature-, pH-, and protein content-dependent conversion of stable T4 to rT3 was found in fetal membranes. Iodothyronine metabolism did not occur in the absence of dithiothreitol. These studies indicate that human fetal membranes contain an inner ring deiodinase enzyme. Because of its intimate contact with the amniotic cavity, this enzyme may generate a portion of the rT3 found in amniotic fluid.

Inhibition of rat hepatic thyroxine 5'-monodeiodinase by propylthiouracil: relation to site of interaction of thyroxine and glutathione

When rat liver cytosol, dialyzed free of glutathione, was chromatographed on Sephadex G-100 after incubation with 35S-propylthiouracil, 2 peaks of bound radioactivity were observed, 1 of which contained nearly all the thyroxine 5'-monodeiodinase activity in rat liver cytosol. Binding of propylthiouracil to this peak was inhibited by glutathione but not by thyroxine. Approximately 25% of 35S-propylthiouracil initially bound to the thyroxine 5'-monodeiodinating activity peak remained bound after dialysis, precipitation with trichloroacetic acid, and multipe extractions with ethanol, methanol, and chloroform, suggesting that binding was at least in part covalent. Dialysis studies showed that the presumed covalent binding of 35S-propylthiouracil to the thyroxine 5'-monodeiodinase peak could be inhibited by glutathione, dithioerythritol, and unlabelled propylthiouracil but not by oxidized glutathione or thyroxine. Conversely, thyroxine binding was unaffected by thiol compounds. We studied the kinetics of thyroxine 5'-monodeiodination by radioimmunoassay techniques using rat liver homogenates as source of enzyme and observed the dependence of enzymic reaction upon glutathione (Km = 2.4 mM). Propylthiouracil inhibited the reaction and this inhibition could be overcome with increasing glutathione concentrations. We conclude that the thiol-dependent thyroxine 5'-monodeiodinase is inhibited by propylthiouracil through its covalent binding, probably as mixed disulfide, to s site on the enzyme at which glutathione interacts either as a cosubstrate or reducing agent. This binding site is separate from the site at which thyroxine binds.

Substrate requirement for inactivation of iodothyronine-5'-deiodinase activity by thiouracil

Preincubation of rat liver microsomal fraction with 1 microM 2-thiouracil and 0.01-1 microM 3,3',5'-triiodothyronine or 3',5'-diiodothyronine, 0.1-10 microM thyroxine or 3,5-diiodothyronine led to a progressive, irreversible and concomitant decrease in subsequently assayed 3,3',5'-triiodothyronine- and 3',5'-diiodothyronine-5'-deiodinase activity. Preincubation with thiouracil alone, with iodothyronines alone or with thiouracil and 10 microM thyronine or 3,5-diiodotyrosine had no or virtually no effect. The results indicate that (1) a previously proposed ping-pong mechanism for thyroid hormone deiodination, involving the formation of an enzyme-sulphenyl iodide intermediate, is correct; (2) thyroxine, 3,3',5'-triiodothyronine and 3',5'-diiodothyronine are substrates for a common 5'-deiodinase; (3) this 5'-deiodinase is not fully specific as regards the position of the iodine substituents in the substrate, since it also appears to catalyse the 5-deiodination of 3,5-diiodothyronine.